I took a look at the 1860 U.S. census for you, for Fond du Lac county. There was no John Lietz or Leitz or Leets there at that time. There was only an Adam Leets, living with his wife and no children in Rosendale. I'm not sure whether the John Lietz/Leitz you're referring to in the later 1880 census was the one married to Henrietta (both born Prussia) with 8 kids (Emma and August, the oldest). Or the other 1880 Rosendale John Lietz/Leitz, 24-year-old son of Jacob. Either way, the birthplaces and ages of those two Lietz families would indicate that those families were still in Prussia in 1860.
Hope that helps.